QPF IN WEST INCLUDING THE FRONT RANGE OF THE ROCKIES

CHARACTERISTICS DIFFER BY SEASON IN SUMMER….SCALE OF THE EVENTS IS VERY

SMALL. THE SCALE IS TIED TO THE MEAN RH AND FORCING LOOK FOR VORTS IN WATER VAPOR IMAGERY LOOK FOR MOISTURE SURGES ON SSMI AND GOES

PW IMAGES IN WINTER, FORCING AND AREAS OF HEAVY RAIN

ARE MUCH MORE DEPENDENT ON OROGRAPHY.

666

7

77

7

7

7

7

77

7

6 6

6

6

6

6

66

6

6

6

6

6

6

6

6

6

6

6

6

66

6

6 6

6

5

6 66

6

99

9

95

5

555 5

55

5

5

5

5

55

5

5

5

5

5

5

5

5

55

5555

3

333

3

3

3

3

53

33

33

3

3

33

33

3

66

3

5

66

44

4

4

4 4

44

4

4

8

33

42

2

33

8

6

8

6

INTERMOUNTAIN PLATEAU SUB-REGION (83 EVENTS)

NORTHWEST COASTAL SUB-REGION (5 EVENTS )

FRONT RANGE SUB-REGION (49 EVENTS)

5

1 1

4

# TYPE CASES

2

1

3

4

5

6

7

8

9

10

SYNOPTIC SCALE TROPICAL

1

SQUALL LINE

SINGLE CLUSTERED

MCS ALPHA

MCS-BETA CIRCULAR

MCS-BETA LINEAR

SYNOPTIC SCALE CYCLONIC CIRCULATION

LARGE SCALE OVERRUNNING

REGENERATIVE10

COMBO OF TWO OR MORE SYSTEMS PASSING OVER THE SAME AREA WITHIN 24 HRS

2

2

28

13

32

41

10

4

4

MAP OF LOCATION AND TYPE OF SYSTEM FOR 137 HEAVY CONVECTIVE RAINFALL EVENTS

FROM CHAPPELL COMET NOTES

LOCATIONS OF LARGEST 13 MCS AND MCC SYSTEMS USED TO PREPARE 4

TYPES OF ATMOSPHERIC COMPOSITES

11

1

111

144

4444

4

22

2 22

2 2

2

2

2

2

22

2

2

221

1

1

1

1

1

11

33

33

LOCATION OF COMPOSITE CASES

3

LEGEND SYNOPTIC TYPE1

4

3

2

BLOCKING ANTICYCLONE

DEFORMATION ZONE

7

6

1715

SHORTWAVE TROUGH IN NORTHWEST FLOW

SHORTWAVE TROUGH IN ZONAL FLOW

#EVENTS (45)

FROM CHAPPELL COMET NOTES

FREQUENCY OF FLASH FLOODING OR 2”/24HR RAINFALL FOR 137 EVENTS IN WEST

NOTE THE HIGH FREQUENCY IN LATE JULY AND AUGUST

0

5

10

15

20

25

30

35

FR

EQ

UE

NC

Y O

F O

CC

UR

RE

NC

E

J F M A M J J A S O N D

SEMI-MONTHLY TIME OF YEAR

0

10

20

30

40

50

60

PE

RC

EN

T O

F C

AS

ES

N-4PM 4-8PM 8PM-M M-4AM 4-8AM 8-AM-N

LOCAL TIME OF OCCURRENCE

FROM CHAPPELL COMET NOTES

HEAVY RAIN EVENTS ALONG THE FRONT RANGEBIG THOMPSON, FORT COLLINS, CHEYENNE, MADISON COUNTY (VA)

•A LARGE AMPLITUDE NEGATIVE-TILTED UPPER RIDGE AXIS LIES JUST EAST AND NORTH OF THE THREAT AREA,

•WINDS ALOFT ARE LIGHT AND SOUTHEASTERLY

•A WEAK SHORTWAVE ROTATES NORTHWARD TOWARDS THE THREAT AREA RESULTING IN WEAK PVA

•A SLOW MOVING FRONT THAT HANGS UP JUST SOUTH OF THE AREA

FROM MADDOX ET AL., 1977

*******CAN ALSO BE CALLED BLOCKING ANTICYCLONE *******

A NARROW BAND OF MOIST UNCONDITIONALLY UNSTABLE AIR JUST BEHIND THE FRONTTHE MOISTURE USUALLY EXTENDS TROUGH A DEEP LAYER (TO 300 MB)PWS 150-200% OR NORMALOROGRAPHIC LIFT PROVIDES MECHANISM TO RELEASE INSTABILITY

300

500

300

SFC

-319

-68

63

512

79 1015.3

PW=1.18” (181% OF NORMAL)

RH=66% (SFC-500 MB)LI=-5

K=37

AVERAGE SOUNDING FOR BLOCKING ANTICYCLONE

Times when the surface dewpoints extend westward to the front range should be monitored very closely

especially when the low-level winds are easterly or southeasterly

FROM CHAPPELL COMET NOTES

H

L

LLIDEALIZED SURFACE PATTERN

Cells develop east of highest terrain+ Cells move slowly north and northwest

+ redevelopment on southeast or southern flank+heaviest rain falls over a very small area

THERMAL AXIS

AND MOISTURE TONGUE

ADOPTED FROM MADDOX ET AL., 1977

500 MBTROF

LOW LEVEL

LOW LEVEL JET

T-Td6oC

THREAT AREA

Td65oF

The vast majority of front range events occur during the late July and early August,

0

5

10

15

20

NU

MB

ER

OF

CA

SE

S

J F M A M J J A S O N DSEMI-MONTHLY TIME OF YEAR DISTRIBUTION

FRONT RANGE EVENTS (49)DIVIDING INTO HALF-MONTH PERIODS

0

20

40

60

80

PE

RC

EN

T O

F C

AS

ES

8AM-2PM 2PM-8PM 8PM-2AM 2AM-8AM

LOCAL TIME OF DAY

and take place during the late afternoon and early evening (2 PM-8 PM)

FROM CHAPPELL COMET NOTES

H20

24

H

L

ETA MODEL FORECAST OF SFC AND 1000-500 MB THICKNESS. USING THESE FORECASTS AND THE THE MAPS TO FOLLOW, PREDICT WHERE

THE HEAVIEST RAIN WILL FALL.

H

L16

20

24

30

16L11

12 HR V.T. 00Z 29 JUL 1997 24 HR V.T. 12Z 29 JUL 1997

70oF+ dewpoints

60oF+ dewpoints

70oF+ dewpoints

H

SURFACE DEWPOINTS ARE FORECAST TO BE IN THE 60S ACROSS EASTERN COLORADO. YOUR IN THE LAST WEEK OF JULY. ARE ANY BELLS GOING OFF?

ETA 500H AND VORTICITY FORECASTSNOTE THE TILT OF THE UPPER RIDGE AXIS.

L

CONFLUENT 500 FLOW

NEGATIVE TILTING RIDGE AXIS

12 HR V.T. 00Z 29 JULY 1997 24 HR V.T. 12Z 29 JULY 1997

OBSERVED MAPS VALID 00Z JULY 29

193

171

185

179

193

173

207

178

190

72

71

7254

72 7082

63

51

7158

6863

63

7364 13

7760

205

54

50 0 13

5

173

.. 1695

7666

6255 65 13

8267

193

7164

74

6848

6178

8270

8365

8270

.. 7169

13114882

63

6446

6556

8566

20

16

SURFACE 850 MB 500 MB

BOUNDARY?

Td15oCT-Td6oC

FROM THESE MAPS WHAT CAN BE INFERRED ABOUT THE PRECIPITATION EFFICIENCY OF ANY CELLS THAT FORM?

WHAT ABOUT THE LCL AND THE PROBABLE DEPTH OF THE WARM LAYER?

MODELS’ 12-36 HR FORECAST OF

ACCUMULATED PRECIPITATION

.01”

.50”

1.0”

.2.0”

.3.0”

ETANGMAVN

WHICH MODEL DO YOU THINK HAS THE BEST FORECAST OF THE SCALE OF THE 2.00” OR GREATER AMOUNTS? WHAT ABOUT THE LOCATION OF THE MAXIMUM RAINFALL?

HOW DID YOU DO?

.50”

1.0”

2.0”

3.0”.

VERIFYING ANALYSIS VALID 12Z JULY 29

8-10” OF RAIN ON FORT COLLINS & NEARBY FOOTHILLS

4 MILES AWAY ONLY .83” OBSERVED

295 HOUSES OR MOBILE HOMES DESTROYED, 4 KILLED

A LARGER MCS MOVED SOUTHEASTWARD AWAY FROM A SMALLER SCALE QUASI-STATIONARY CONVECTIVE STORM.

SUMMERTIME QPF IN THE WEST AND ALONG THE FRONT RANGE INVOLVES

DETERMINING WHETHER CONDITIONS WILL BE CONDUCIVE TO HEAVY RAINS

IS STRONGLY RELATED TO THE MONSOON SEASON

THE SCALE IS USUALLY TOO SMALL TO ALLOW A FORECAST TO REALISTICALLY PREDICT THE EXACT LOCATION WITH MUCH LEAD TIME.

THE SCALE ARGUES FOR A PROBABILISTIC APPROACH TO THE PROBLEM

MORE ON SUMMERTIME WESTERN EVENTS

0

5

10

15

20

25

NU

MB

ER

OF

CA

SE

S

J F M A M J J A S O N DSEMI-MONTH TIME OF YEAR

INTERMOUNTAIN PLATEAU (83 EVENTS)

EVENTS IN INTERMOUNTAIN REGION ALSO HAVE A DISTINCT MAXIMUM DURING THE 6-HR PERIOD BETWEEN 2 PM AND 8 PM LOCAL DAYLIGHT TIME.

OCCUR MOSTLY IN AUGUST AND SEPTEMBER INTO EARLY OCTOBER.

Southwest, blocking high type heavy rainfall event.

500 HEIGHTS AND VORTICITY

10

588

582 582576

588

H

L

14

08

08

06

06

10

10 12

T-T d6o C

TROF

TROF

700 MB COMPOSITE

L

2oC

312

0oC

4oC6oC8o

315

315

-2oC H

L315 Td

Note that the high center shifts east of 4-corners. This allows moisture to be pulled northward from the Gulf of CA.

Note there is often a upper low to the southwest with a vorticity lobe extending towards the threat area. These features often show up best on water vapor imagery.

6389

6

12

6

-6

13

Blocking anticyclone type (also is known as Maddox Type 4)

-31

L

10161016

1020

1020

10081012

1024

1024

H50o

50o

55o

60o

65o+

300

500

700

SFC

(133% OR NORMAL)

PW=1.32”

LI=-2

RH=58%

KI=37

SURFACE COMPOSITE 1016

NOTE WEAK MEAN FLOW AND RELATIVELY DEEP MOISTURE. SCALE OF THE EVENT IS SMALL.

DEFORMATION TYPE HEAVY RAINFALL EVENT

576

L

570 582

588

594

04

0810

12

L10

T-Td6oC

500 HEIGHTS AND VORTICITY

318

315

312

309L

4o

2o

0-2o

-4o

-6o

-8o

2o0

-2o

700 HEIGHTS AND DEWPOINTS

STRONG UPPER LEVEL DIVERGENCE IS PRESENT, OFTEN ENHANCED BY ENTRANCE REGION OF UPPER LEVEL JET STREAK

An old front or surface boundary and weak mean flow are usually present

1012

10101010

1012

1012

10121008

1008

1016

1016

65o

60o

50o

50o

60o

65o

300

500

700

SFC

-33

-85

9

810

8159

1010.7

PW=1.15”

(160% OF NORMAL)

RH=66%

LI=-2KI=35

QPF IN WEST DURING WINTER

OVERALL SCALE IS LARGER THAN IN SUMMER SCALE AND POSITION OF HEAVIEST RAIN IS

OFTEN CLOSELY TIED TO OROGRAPHY SATELLITE IMAGERY, SSMI AND GOES PW

PRODUCTS ARE GREAT TOOLS TIMES WHEN AXIS OF DEEP MOISTURE IS QUASI-

STATIONARY OFTEN PRODUCE 3 INCH OR GREATER AMOUNTS IN THE CASCADE AND SIERRA RANGES.

IN WEST IN WINTER CONT.

USUALLY THERE IS A SIGNIFICANT RAIN SHADOW TO THE LEE OF THE MOUNTAINS.

MOUNTAIN MAPPER AND THE RHEA OROGRAPHIC TECHNIQUE CAN COMBINE TO PROVIDE A VERY GOOD FIRST GUESS.

WHEN MOISTURE IS UNUSUALLY DEEP RAIN, HEAVY RAIN WILL FALL ON THE LEEWARD SIDE OF THE SIERRA RANGE IN EASTERN NV AND WESTERN CA

SIGNIFICANT SKILL IN 24 HOURS FORECASTS, SKILL IS CONSIDERABLY LOWER FOR 6 HOURLY FORECASTS

MADDOX ET AL., WINTER TYPE III

MOST COMMON IN WINTER AND EARLY SPRING STRONG UPPER TROF SWEEPING INTO WEST COAST FROM

PACIFIC HEAVY PRECIPITATION ENDS AS UPPER TROF AXIS NEARS

COAST. THE STRONGER THE LOW LEVEL WINDS NORMAL TO THE

MOUNTAINS, THE HEAVIER THE RAIN THERE IS USUALLY A BLOCK OVER ALASKA OR THE GULF

OF ALASKA LOOK FOR TROPICAL CONNECTIONS

200

300

Maddox et al., Type III

500

700

SFC

-45

-19-12

-6

-2

5850

1011

PW=.72”

(159% OF NORMAL)

KI=27

-53

At the end of the event, comprised about 20% of flash floods. In reality, risk area is bigger than shown.

Type III events typically occur in southern California when a block forms over Alaska and the Gulf of Alaska and the westerlies break through to

its south.

A number of shortwaves then affect the state and produce a prolonged period of heavy rains.

BL

OC

K

NEED TO KNOW TERRAIN AND WIND

DIRECTION THAT FAVORS UPSLOPE

LA

GU

NA

MT

NS

L.A.

AREA

SIERRA MADRE MTNS

SAN GABRIEL MTNS

CATALINA I.

SANTA BARBARA

SAN DIEGO.

POPULATION CENTERS

MOUNTAIN RANGES

5”

10”

15”

15”

15”

20”

25”30”

25”

SCALE 50 MILES

SANJACINTO

MTNS

SAN

BERNARDINOMTNS

5”

9-DAY TOTAL RAINFALL (6 SEPARATE STORMS) . HEAVY RAINFALL EVENTS IN WINTER ALONG THE WEST COAST OFTEN ARE ASSOCIATED WITH MULTIPLE STORMS.

A TYPE III CASE STUDY

MSL, THICKNESS AND 850 WINDS

VALID AT BEGINNING OF PERIOD

PRECIPITABLE WATER (INCHES) AND 850 MB WINDS

A COUPLE THINGS TO NOTE: 1) A LONG FETCH OF DEEP MOISTURE, 2) A BARRIER JET AND STRONG SOUTHERLY FLOW UP THE SACREMENTO VALLEY. THIS JET HELPS PRODUCE HEAVY RAINS NEAR SHASTA

DO THE FORECAST LOOK CONSISTENT WITH WHAT YOU SEE IN PACIFIC?

MSL, THICKNESS AND 850 WINDS PRECIPITABLE WATER (INCHES) AND 850 MB WINDS

VALID AT THE END OF THE PERIOD

OVERLAYING MODEL OUTPUT WITH SSMI IMAGERY CAN GIVE YOU A GOOD IDEA OF THE MOISTURE THAT WILL BE FEEDING INTO THE WEST COAST.

THE MODEL OUTPUT LOOKS REASONABLE

NOTE THE TROPICAL CONNECTION AND PLUME OF PWS ABOVE 1.00”

CAN LOOK AT MODEL OUTPUT FOR LIFTING DUE TO TERRAIN

BOUNDARY LAYER MOISTURE TRANSPORT AND VERTICAL FLUX OF MOISTURE DUE TO OROGRAPHY

VALID 18Z 8 JAN. 1995 VALID 12Z 10 JAN. 1995

NOTICE HOW SLOWLY THE LIFTING SHIFTS SOUTH

MODEL PRECIPITATION IS INCORRECTLY SHIFTED AWAY FROM THE MOUNTAINS

BECAUSE OF THE SIMPLIFIED MICROPHYSICS

– THE MODELS DO NOT PREDICT THE HORIZONTAL ADVECTION OF PRECIPITATON (DENDRITES AND NEEDLES CAN ADVECT SIGNIFICANT DISTANCES FROM WHERE THEY FORM)

AND INADEQUATE RESOLUTION OF MOUNTAINS.

– THE TERRAIN IS AVERAGED SO THE SLOPE OF THE MOUNTAINS ARE NOT STEEP ENOUGH

WITH STRONG VERY MOIST SOUTHWESTERLY FLOW AT 850 AND 700 MB NOTE HOW CLOSELY THE

PRECIPITATION CONFORMS TO THE TERRAIN

24 HOUR PRECIPITATION VALID JAN. 9, 1995

WHEN SOUTHWESTERLY FLOW IS PRESENT A BARRIER JET FORMS AND FUNNELS THE FLOW UP THE SACREMENTO VALLEY. KNOWING THE CLIMATE AND TERRAIN ARE AN IMMENSE HELP WHEN FORECASTING PRECIPITATION AMOUNTS

PRISM JANUARY AVERAGE CLIMATOLOGICAL PRECIPITATION

IN SUMMARY DURING SUMMER OVER INTERMOUTAIN REGION

AND MOST OF THE WEST, QPF IS ALMOST IMPOSSIBLE BECAUSE – THE MODELS DO A BAD JOB HANDLING THE MOISTURE– THE SCALE OF EVENTS IS USUALLY SMALL

DURING WINTER, THE SKILL OF QPF IS HIGHER OVER NORTHERN CA, WESTERN OR AND WESTERN WA THAN ANYWHERE ELSE IN THE COUNTRY . PATTERN RECOGNITION WORKS VERY WELL!– BECAUSE THE FOCUS FOR MOST OF THE LIFTING IS

STATIONARY (THE MOUNTAINS).– HOWEVER TIMING WHICH 6 HOUR PERIOD WILL

RECEIVE THE HEAVIEST RAINFALL IS STILL DIFFICULT.